This invention relates to a metal vapor discharge lamp including a translucent heat-resistant, corrosion-resistant ceramic tube as a light emitting tube with both the ends thereof sealed with ceramic sealing members.
A metal vapor discharge lamp, such as a high-pressure sodium lamp, includes a light emitting tube or inner burner of a translucent ceramic of a kind which has a resistance to sodium, such as alumina ceramics, discharge electrodes are attached to the ceramic, and it is filled with a starting rare gas and a sodium amalgam. The translucent ceramic tube is a straight one usually having a uniform diameter. Since, however, both the open ends of the tube cannot be melt-sealed as in the case of a quartz glass tube, they are hermetically sealed by the corresponding sealing members made of ceramic material which is the same as the ceramic material of the light emitting tube. The discharge electrodes are supported by the corresponding sealing members, respectively. The light emitting tube is sealed within an outer glass envelope equipped at one end with a base to which a lead-in wire from the light emitting tube is connected. Vacuum is usually maintained within the outer glass envelope. The high-pressure sodium lamp so formed finds a wider acceptance as a light source for an energy saving. With this trend, various types of such lamps are now being developed.
In the high-pressure sodium lamp, a condensation of sodium amalgam often occurs on the end of the light emitting tube of alumina ceramics during the extinguishing of the lamp. In this case, upon the starting of the lamp, a discharge arc spot may be formed on this sodium amalgam in place of on the electrode. This phenomenon is what is called a "back arc" phenomenon. The occurrence of this phenomenon abnormally heats the ceramic portion of the tube, causing cracks thereon and a consequent lamp failure.
In order to solve the above-mentioned problem, a means is disclosed in Japanese Patent Publication No. 49-12981 whereby a difference between the outer diameter of a discharge electrode and the inner diameter of a light emitting tube made of ceramics is made small. In a high power output (for example, 700 W, 1,000 W) type lamp and high color-rendering type lamp, use is made, of a ceramic tube of a relatively great diameter as a light emitting tube. For example, an inner-diameter of about 10 mm or 14 mm can be used to obtain a high efficiency and high color-rendering property. Such a lamp is readily subject to the above-mentioned "back arc" phenomenon. However, if the invention of Japanese Patent Publication No. 49-12981 is applied to a light emitting tube of greater inner diameter, it is necessary to increase the outer diameter of the associated electrodes. The electrode dimension is determined by a lamp characteristic, for example, the lamp current, and has no direct dependency upon the inner diameter of the light emitting tube. If the electrode dimension is caused to increase depending upon the inner diameter of the light emitting tube without paying attention to the above-mentioned restrictions, then the starting characteristic of the lamp is degraded and/or an electron emissive material is abnormally sputtered from the electrode during the lighting of the lamp, causing the occurrence of blackening on the tube and portion of the light emitting tube. The blackening phenomenon results in a lowering in the lamp luminous flux, an abnormal rise in the lamp voltage and a consequent degradation in the expectant life characteristic. In these respects, the technique of Japanese Patent Publication No. 49-12981 is insufficient for a lamp having a large-diameter light emitting tube.
U.S. Pat. No. 3,932,782 discloses the prior art of the end configuration of the light emitting tube. In this patent, materials added within the light emitting tube are readily deposited on the boundary portion between a tubular body portion and an end portion of the light emitting tube. For a large-diameter tube, the additive may be condensed in a position away from the electrode, thus producing a back arc phenomenon due to the arc spot formed on the additives, such as sodium amalgam.
For example, Japanese Utility Model Publication No. 51-1641 discloses a method for controlling the coolest temperature on the tube end section by varying the configuration of sealing members for use at both the ends of a light emitting tube. This method, however, involves a high processing cost and a greater heat loss at the tube end section.